The present invention relates to a keying circuit for identifying each of manually operated keys and detecting their operating states, and has for its object the provision of such a keying circuit which scans an array of key switches to identify the operated key by comparing a key identifying voltage with a variable reference represented by a datum stored in memory. In particular, the circuit eliminates disadvantages arising from key identifying voltages having indefinite amplitudes which are generated by the gradually increasing contact resistance at the instant the key switch is released into a turn-off state.
A known microprocessor-based keying circuit includes a voltage divider having a plurality of series-connected resistors with taps between successive resistors. An array of manually operated non-locked key switches are connected respectively to the taps to provide a plurality of graduated voltages respectively indentifying the particular functions of the key switches. When any one of the key switches is operated, a corresponding key identifying voltage is applied to the noninverting input of a comparator whose inverting input is supplied with a reference voltage of staircase waveform which is recyclically generated under the control of a microcomputer. When the two input voltages match, the comparator generates a coincidence signal and enables the microcomputer to store a datum representing the instantaneous value of the reference staircase voltage that occurred at the instant of the match into a memory. The same process is repeated to count the number of occurrences of match between the two input voltages. When this count value reaches a predetermined value the microcomputer verifies it as a valid indication that a key has been operated into a turn-on state and provides a corresponding mode selecting signal to a video recorder or the like to operate it in a mode specified by that key.
The keying circuit of this type is known as a scan mode discriminator because the time-varying reference voltage is used to sequentially scan the voltage level of the signal given by the operated key. The key scanning operation continues during turn-on periods so that at the instant the key is released the microcomputer detects a change in voltage supplied from the key array.
This type of keying circuit is advantageous for applications where the key array is mounted as a separate unit in a location remote from the circuit that identifies the key functions because it requires a small number of wires interconnecting the separated units. However, a disadvantage arises from the fact that a voltage of indefinite amplitude is likely to develop across the contact resistance of the key being turned off if it remains in an ambiguous position between turn-on and turned-off states. It is therefore highly likely that the microcomputer falsely identifies the indefinite voltage as an indication of the operation of a different key and generates a false mode selecting signal.